Many work machines are required to operate on uneven terrain or in other poor footing conditions. To provide better traction, work machines such as motor graders are often equipped with an all-wheel-drive (AWD) system. A typical AWD vehicle has rear wheels driven to propel the vehicle and front wheels used for steering. In a poor footing condition, the front wheels of the AWD vehicle can also be driven to increase traction of the vehicle. For example, a motor grader may be operated in an AWD mode to obtain maximum traction when grading on a side slope or removing snow from a roadway.
To avoid unnecessary wear of AWD components and to minimize power loss, AWD vehicles can be operated in a two-wheel-drive mode. When operating a motor grader, it may be desirable to propel the motor grader only by its front wheels at a slow speed so that its rear wheels do not disturb the fine finished graded surface. Such an operating mode in a motor grader is typically called a creep mode.
In the creep mode, however, it is difficult to control the speed of the vehicle. A known motor grader allows an operator to control the speed in the creep mode by varying engine speed (RPM). For example, U.S. Pat. No. 6,508,328 discloses a motor grader that controls vehicle speed in a creep mode based on engine speed control by an operator. However, these known motor graders do not control the ground speed independently of the engine speed, and they do not allow the engines to operate at full capacity. Furthermore, those motor graders require that an operator perform a difficult task of modulating engine speed by an accelerator pedal. As a result, those AWD vehicles can be operated inefficiently.
The present control system is directed to solving one or more of the shortcomings associated with prior art designs and providing a system and method for controlling an AWD vehicle with better control and efficiency.